Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session J7: Effects of Collisions |
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Chair: Robert Forrey, Penn State University Room: 553AB |
Wednesday, May 25, 2016 2:00PM - 2:12PM |
J7.00001: Nuclear Thermal Motion Driven Adiabatic Electron States Thermalization and the Induced Phase Transition From Insulator To Metal in Warm Dense Matters Dongdong Kang, Yong Hou, Cheng Gao, Jiaolong Zeng, Jianmin Yuan In warm dense matter(WDM), the thermal motion energy of a nucleus may be comparable to its coupling energy with the neighbor nuclei and comparable to the valence electronic orbital motion energy. As the much small mass of electrons, the fluctuations of the electron orbitals are almost adiabatic dynamical changes with nuclear motion. The electronic and nuclear structure of warm and dense He and Ar are simulated by using the density functional based molecular dynamics method. The nuclear thermal motion driven adiabatic thermalization of the electron states, depression of the energy band gap and even phase transitions of electron states from insulator to metal are predicted when the nuclear thermal motion energy is comparable to the coupling energy between the neighboring atoms as well as to the valence electronic orbital motion energy. These kind of nuclear thermal motion driven adiabatic electronic states from insulator to metal phase transition occurs at much lower temperatures than the normal thermal collision excitation in low density and high temperature gases. [Preview Abstract] |
Wednesday, May 25, 2016 2:12PM - 2:24PM |
J7.00002: New Interference Mechanism Controls Ultracold Chemistry Brian K. Kendrick, Jisha Hazra, N. Balakrishnan A newly discovered interference mechanism has been shown to control the outcome of ultracold chemical reactions. The mechanism originates from the unique properties associated with ultracold collisions, namely: (1) isotropic (s-wave) scattering and (2) an effective quantization of the scattering phase shift (which originates from the bound state structure of the molecule). These two properties can lead to maximum constructive or destructive interference between two interfering reaction pathways (such as exchange and non-exchange in systems with two or more identical nuclei). If the molecular system exhibits a conical intersection, then the associated geometric phase is shown to act as a ``quantum switch'' which can turn the reactivity on or off. Reaction rate coefficients for the O + OH $\rightarrow$ H + O$_2$ and H + H$_2$, reactions are presented which explicitly demonstrate the effect. Experimentalists might exploit this new mechanism to control ultracold reactions by the application of external electric or magnetic fields or by the selection of a particular nuclear spin state. [Preview Abstract] |
Wednesday, May 25, 2016 2:24PM - 2:36PM |
J7.00003: Magnetic-field Control of the Ultracold Chemistry of Simple Molecules Brandon Ruzic, Jisha Hazra, Naduvalath Balakrishnan, John Bohn New experimental techniques continue to bring new molecular species into the ultracold regime. At these temperatures, modest external fields can significantly affect collisions. We theoretically explore the ultracold quantum chemistry of simple molecules, including the benchmark chemical reaction of F + H$_2$. We find that resonances that exist in the van der Waals wells of entrance channels can be manipulated by magnetic fields, thus affecting the ability of the reactants to tunnel through the chemical barrier. We focus on the possibility of ultracold Fano-Feshbach resonances and their ability to affect the distribution of product states. [Preview Abstract] |
Wednesday, May 25, 2016 2:36PM - 2:48PM |
J7.00004: Full-dimensional close-coupling study of rovibrationally inelastic scattering of SiO-H$_2$ B. Yang, X. Wang, P. Zhang, P. C. Stancil, J. M. Bowman, N. Balakrishnan, R. C. Forrey Molecular collisional excitation rate coecients are required to interpret spectra of molecular gas not in local thermodynamic equilibrium. Silicon monoxide (SiO) has been detected in a variety of astronomical sources and is of astrophysical importance. Its rovibrational level populations are perturbed by collisions with He, H and H$_2$. The corresponding collisional rate coefficients and their temperature dependence are largely unknown. Theoretical scattering calculations are the primary source of such rate coefficients. In this work a full-dimensional (6D) potential energy surface (PES) of SiO-H$_2$ was calculated using the high-level CCSD(T)-F12B method and fitted using an invariant polynomial approach in 6D. We performed the first full dimensional quantum close-coupling scattering calculations for SiO in collision with H$_2$ on the 6D PES. Pure state-to-state rotational excitation transitions from SiO($v_1=0$, $j_1$=0-10) are computed. For rovibrational transitions, state-to-state and total quenching cross sections and corresponding rate coefficients from several low-lying rotational levels in the first excited vibrational level of SiO are calculated for both para-H$_2$ and ortho-H$_2$ collisions. [Preview Abstract] |
Wednesday, May 25, 2016 2:48PM - 3:00PM |
J7.00005: An effective field theory analysis of Efimov features in heteronuclear mixture of ultracold atomic gases Bijaya Acharya, Chen Ji, Lucas Platter Recent experimental studies have unveiled Efimov physics in ultracold atomic gases of heteronuclear mixtures. The recombination features of such atomic systems display universal correlations including discrete scaling invariance. We use Effective Field Theory (EFT) to study the Efimov features of the heteronuclear three-atom systems consisting of two identical bosons which interact with each other through a natural scattering length and with the third particle through a large scattering length. We compute the corrections to the universal correlations by perturbative insertions of the interspecies effective range and the intraspecies scattering length. Such an analysis is relevant for mixtures of ultracold atomic gases near the interspecies Feshbach resonance. [Preview Abstract] |
Wednesday, May 25, 2016 3:00PM - 3:12PM |
J7.00006: Surpassing the mass restriction of buffer gas cooling: Cooling of low mass ions by localized heavier atoms Sourav Dutta, Rahul Sawant, S. A. Rangwala Cooling of trapped ions has resulted in fascinating science including the realization of some of the most accurate atomic clocks. It has also found widespread application, for example, in mass spectrometry and cold chemistry. Among the different methods for cooling ions, cooling by elastic collisions with ultracold neutral atoms is arguably the most generic. However, in spite of its widespread application, there is confusion with regards the collisional heating/cooling of light ions by heavier neutral atoms. We address the question experimentally and demonstrate, for the first time, cooling of light ions by co-trapped heavy atoms [1]. We show that trapped $^{39}$K$^{+ }$ ions are cooled by localized ultracold neutral $^{85}$Rb atoms. The atom-ion mass ratio (= 2.18) is well beyond any theoretical predictions so far. We further argue that cooling of ions by localized cold atoms is possible for any mass ratio. The result opens up the possibility of reaching the elusive s-wave collision regime in atom-ion collisions. [1] S. Dutta et al., Collisional cooling of light ions by co-trapped heavy atoms, \textit{arXiv}: 1512.04197 [Preview Abstract] |
Wednesday, May 25, 2016 3:12PM - 3:24PM |
J7.00007: The magnetic toroidal sector: a broad-band electron-positron pair spectrometer Siegbert Hagmann, Pierre-Michel Hillenbrand, Yuri Litvinov, Uwe Spillmann At the future relativistic storage-ring HESR at FAIR the study of electron-positron pairs from non-nuclear, atomic processes will be one of the goals of the experimental program with kinematically complete experiments focusing on momentum spectroscopy of coincident emission of electrons and positrons from free-free pairs and corresponding recoil ions. The underlying production mechanisms belong to central topics of QED in strong fields. We present first results on the electron-optical properties of a magnetic toroidal sector configuration enabling coincident detection of free-free electron-positron pairs; this spectrometer is suitable for implementation into a storage ring with a supersonic jet target and covering a wide range of lepton emission into the forward hemisphere. The simulation calculations are performed using the OPERA code [1]. \begin{enumerate} \item OPERA-3D, Vector Fields Limited, Oxford, UK \end{enumerate} [Preview Abstract] |
Wednesday, May 25, 2016 3:24PM - 3:36PM |
J7.00008: Observation of a power-law energy distribution in atom-ion hybrid system Ziv Meir, Nitzan Akerman, Tomas Sikorsky, Ruti Ben-Shlomi, Yehonatan Dallal, Roee Ozeri Understanding atom-ion collision dynamics is at the heart of the growing field of ultra-cold atom-ion physics. The naive picture of a hot ion sympathetically-cooled by a cold atomic bath doesn't hold due to the time dependent potentials generated by the ion Paul trap. The energy scale of the atom-ion system is determined by a combination of the atomic bath temperature, the ion's excess micromotion (EMM) and the back action of the atom-ion attraction on the ion's position in the trap. However, it is the position dependent ion's inherent micromotion which acts as an amplifier for the ion's energy during random consecutive collisions. Due to this reason, the ion's energy distribution deviates from Maxwell-Boltzmann (MB) characterized by an exponential tail to one with power-law tail described by Tsallis q-exponential function. Here we report on the observation of a strong deviation from MB to Tsallis energy distribution of a trapped ion. In our experiment, a ground-state cooled $^{88}$Sr$^+$ ion is immersed in an ultra-cold cloud of $^{87}$Rb atoms. The energy scale is determined by either EMM or solely due to the back action on the ion position during a collision with an atom in the trap. Energy distributions are obtained using narrow optical clock spectroscopy. [Preview Abstract] |
Wednesday, May 25, 2016 3:36PM - 3:48PM |
J7.00009: Radiative double electron capture (RDEC) by bare fluorine ions on a nitrogen target.* Nuwan Kumara, David La Mantia, Asghar Kayani, Anna Simon, John Tanis Unlike radiative electron capture (REC), in which a single photon is emitted due to capture of a single electron from the target to the projectile, radiative double electron capture (RDEC) involves two electrons accompanied by the emission of a single photon. Hence, RDEC can be considered as the inverse of double photoionization and used to study the role of electron correlation in causing the process. We report recent results obtained for 40 MeV F$^{\mathrm{9+}}$ ions incident on a nitrogen target, in which counts were observed in the calculated RDEC region (2.8-4.4 keV) for the system. Based on these observations an approximate value for the total RDEC cross section was estimated. Compared with the data obtained for 38 MeV O$^{\mathrm{8+}}$ ions incident on a carbon foil target$^{\mathrm{1}}$, the present value is considerably smaller than the value found for carbon, but in better agreement with recent theory$^{\mathrm{2}}$. *Supported in part by NSF. $^{\mathrm{1}}$A. Simon et el., PRL \textbf{104}, 123001 (2010); $^{\mathrm{2}}$ E. A. Mistonova et el., Phys. Rev. A \textbf{87}, 034702 (2013). [Preview Abstract] |
Wednesday, May 25, 2016 3:48PM - 4:00PM |
J7.00010: Single and double capture in $\rm{F}^{9+}$ + Ar collisions: Comparison of total capture with capture occurring from the Ar K shell David La Mantia, Nuwan Kumara, Asghar Kayani, Anna Simon, John Tanis Total cross sections for single and double capture, as well as the corresponding cross sections for capture resulting in the emission of an Ar K x ray, were measured. This work was performed at Western Michigan University with the use of the tandem Van de Graaff accelerator. A 45 MeV beam of fully-stripped fluorine ions was collided with argon gas molecules in a differentially pumped cell. Surface barrier detectors were used to observe the charge changed projectiles and a Si(Li) x-ray detector, placed at $90^{\rm{o}}$ to the incident beam, were used to measure coincidences with Ar K x rays. The total capture cross sections are compared to previously measured cross sections in the existing literature.\footnote{J. H. Houck, et al, Phys. Rev. A {\bf 56}, 1954 (1997)}\textsuperscript{,}\footnote{J. R. Macdonald and F. W. Martin, Phys. Rev. A {\bf 4}, 1965 (1971)} The coincidence cross sections, considerably smaller than the total cross sections, are found to be nearly equal for single and double capture in contrast to the total cross sections, which vary by about an order of magnitude. Possible reasons for this behavior are discussed. [Preview Abstract] |
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